Hydrogels for biomedical applications were developed extensively in the recent years.
They have become an important material of high interest due to their high water content, tissue-like elasticity and rather good biocompatibility.
Hydrogels are composed of hydrophilic homopolymer or copolymer networks and are able to swell in water or physiological fluids. Hyaluronic acid (HA), also called hyaluronan, or its salt sodium hyaluronate, is a natural and linear heteropolysaccharide polymer typically found in the connective and epithelial tissues of vertebrates with a molecular weight range of 105-107 daltons. HA is composed of repeating disaccharide units of β-1, 3-N-acetyl glucosamine and β-1, 4-glucuronic acid with excellent viscoelasticity properties, high moisture retention capacity, and high bio compatibility (Lavertu et al., 2008, Biomacromolecules, 9, 640-65). Commonly, HA was extracted from rooster combs and currently it is essentially produced via microalgae or bacterial fermentation (Liu et al., 2011, Microb. Cell Fact., 10, 99). HA is suitable for a wide range of applications in medicine, cosmetics, and nutraceuticals (Fraser et al., 1997, J. Intern. Med., 242, 27-33; Robert, 2015, Pathol Biol, 63, 32-34). In particular, it is known for its role in the lubrication and homeostasis of cartilage. However, native HA does not persist for a long time in the human body and is cleared from the implant site because of its low retention capacity and its degradation due to the effect hyaluronidase (Brown et al., 1991, Exp. Physiol., 76, 125-134). Therefore, chemical modifications of HA, in particular cross-linking modifications, have been considered for efficient applications (Schante et al., 2011, Carbohydrate Polymers, 85, 469-489). pH or thermo-sensitive polymers were also synthesized, formulated and formed a hydrogel matrix (Na, 2008, Tissue Engineering and Regenerative Medicine, 5, 482-487; Coronado et al., 2011, Polymer Bulletin, 67, 101-124 and Lee et al., 2010, Soft Matter, 6, 977-983). However, these pH or thermo-sensitive polymers, due to their physicochemical properties, only form a gel matrix and do not provide an efficient cushioning effect. Furthermore, their residence time in vivo is still limited by the rapid enzymatic HA degradation.
Poly(N-isopropylacrylamide) (variously abbreviated in the literature pNiPAM, PNIPA, PNIPAAm, NIPA, PNIPAA or PNIPAm, also named after IUPAC poly[1-(isopropylcarbamoyl)ethylene], also named N-(1-Methylethyl)-2-propenamide homopolymer) is a highly pH or thermo-responsive polymer previously used to modify HA to impart pH or thermo-sensitivity (Schild, 1992, Progress in Polymer Science, 17, 163-249). pNiPAM represents a candidate with a good biocompatibility to introduce physical cross-links via association of hydrophobic domains and in situ forming hydrogel (Tan et al., 2009, Biomaterials, 30, 6844-6853; Cooperstein et al., 2013, Biointerphases, 8, 19). In the case of thermo-sensitive below the lower critical solution temperature (LCST) at about 32° C., the hydrophobic N-substituted groups of pNiPAM are hydrated by water molecules to form a homogeneous solution. Above LCST, hydrophobic interaction between the N-substituted groups increases and surpasses the water hydration energy, leading to aggregation of hydrophobic polymer chains and hydrogel formation (Guan et al., 2011, Expert Opinion on Drug Delivery, 8, 991-1007). Even though the pNiPAM monomer is non-biodegradable, it has been demonstrated that low molecular weights of pure pNiPAM chains are eliminated by renal clearance (He et al., 2008, Journal of Controlled Release, 127, 189-207). Thermo-reversible hyaluronic acid-poly(N-isopropylacrylamide) (HA-pNiPAM) hydrogels were already formulated for applications in multiple fields such as tissue engineering, drug delivery system, etc. and form a hydrogel matrix at body temperature similar to pure pNiPAM (Ohya et al., 2001, Biomacromolecules, 2, 856-863). In particular, thermo-reversible hyaluronan-poly(N-isopropylacrylamide) (HA-pNiPAM) hydrogels were synthesized through reversible addition-fragmentation chain transfer polymerization (RAFT) and “click” chemistry, through the functionalization of HA with alkyne groups and the reaction of the so-formed HA propargylamide with azido-terminated Poly(N-isopropylacrylamide) (N3—PNIPAM) in presence of a chain transfer agent (CTA) and a copper-containing catalyst (WO 2010/099818; Mortisen et al., 2010, Biomacromolecules, 11, 1261-1272). Their degradation products were found cytocompatible to hTERT-BJ1 fibroblasts at 35×103 g·mol−1 (Mortisen et al., 2010, supra). The disadvantages of these HA-pNiPAM gel matrixes lie in the fact that they do not provide long-term lubrication or cushioning effect and furthermore can contain residual copper.
Thus, there is a need to find HA-derived polymers that can provide a long residence time in the body at the injection site with appropriated biomechanical properties, while being well tolerated and easy to synthesize.